A 100-sided polygon is called a hectogon, centagon or 100-gon. In general, any n-sided polygon with over 12 sides is called an n-gon. While there are other conventions for naming polygons with any number of sides, it is most common to use the n-gon form.
The term hectogon comes from Greek root hecto-. Other similar terms are hectometer (100 meters) and hectogram (100 grams). Centagon comes from the Latin root cent-, as in century (100 years). Standard names for polygons use both Latin and Greek roots. For example, quadrilateral uses the Latin root quadri-, meaning four, while hexagon uses the Greek root hexa-, meaning six.